Static Random Access Memories and Access Methods Thereof

ABSTRACT

A static random access memory device capable of preventing stability issues during a write operation is provided, in which a memory cell is coupled to a read word line, a write word line, a read bit line, a write bit line and a complementary write bit line, and a multiplexing unit is coupled to the read bit line, the write bit line and the complementary write bit line. The multiplexing unit applies first and second logic voltages representing a logic state stored in the memory cell to the write bit line and the complementary write bit line, respectively, when the memory cell is not selected to be written by an input signal from a data driver and the read word line is activated, in which the first and second logic voltages are opposite to each other.

CROSS REFERENCE

This application is a Divisional of Application No. 12/249,988, filedOct. 13, 2008, the subject matter of which is incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to static random access memories, and moreparticularly, to static random access memories capable of preventingstability issues, such as static noise margin issues, during a writeoperation.

2. Description of the Related Art

Current trends in the semiconductor and electronics industry requirememory devices to be made smaller, faster and require less powerconsumption. One reason for these trends is that more relatively smalland portable personal devices are being manufactured, thereby relying onbattery power. In addition to being smaller and more portable, personaldevices are also requiring increased memory and more computational powerand speed. In light of all these trends, there is an ever increasingdemand in the industry for smaller, faster, and lower power dissipationmemory cells and transistors used to provide the core functionality ofthe memory devices.

Semiconductor memories can, for example, be characterized as volatilerandom access memories (RAMs) or nonvolatile read only memories (ROMs),where RAMs can either be static (SRAM) or dynamic (DRAM) differingmainly, in the manner by which they store a state of a bit. For an SRAM,for example, each memory cell includes transistor-based circuitry thatimplements a bistable latch, which relies on transistor gain andpositive (e.g., reinforcing) feedback so that it can only assume one oftwo possible states, namely an on (state 1) or off (state 2). The latchcan only be programmed or induced to change from one state to the otherthrough the application of a voltage or other external stimuli. Thisarrangement is desirable for a memory cell since a state written to thecell will be retained until the cell is reprogrammed.

On the other hand, DRAMs implement a capacitor that is either charged ordischarged to store the on (state 1) or off (state 2) state of a cell.Capacitors discharge over time, however, and DRAMs must therefore beperiodically ‘refreshed’. Also, a bistable latch can generally beswitched between states much faster than the amount of time it takes tocharge or discharge a capacitor.

SRAMs are a desirable type of memory for certain types of applications.

BRIEF SUMMARY OF THE INVENTION

Embodiments of a static random access memory device are provided, inwhich a first memory cell is coupled to a read word line, a write wordline, a first read bit line, a first write bit line and a firstcomplementary write bit line, and a second memory cell is coupled to theread word line, the write word line, a second read bit line, a secondwrite bit line and a second complementary write bit line. A word linedriving unit is coupled to the read word line and the write word line,and first and second multiplexing units are coupled to the first andsecond the memory cells. The word line driving unit activates the readword line for a first time interval when the second memory cell isselected to be written by an input signal from a data driver and theword line driving unit then activates the write word line in the firsttime interval to write the input signal from the data driver to thesecond memory cell.

The invention provides an embodiment of an access method for a staticrandom access memory device, wherein the static random access memorydevice comprises first and second memory cells driven by a read wordline and a write word line, and the first memory cell is further coupledto a first read bit line, a first write bit line and a firstcomplementary write bit line and the second memory cell is furthercoupled to a second read bit line, a second write bit line and a secondcomplementary write bit line. The method comprises activating the readword line for a first time interval when the second memory cell isselected to be written by an input signal from a data driver; andactivating the write word line to write the input signal from the datadriver to the second memory cell during the first time interval.

The invention provides an embodiment of a static random access memorydevice, in which a memory cell is coupled to a read word line, a writeword line, a read bit line, a write bit line and a complementary writebit line, and a multiplexing unit is coupled to the read bit line, thewrite bit line and the complementary write bit line. The multiplexingunit applies first and second logic voltages representing a logic statestored in the memory cell to the write bit line and the complementarywrite bit line, respectively, when the memory cell is not selected to bewritten by an input signal from a data driver and the read word line isactivated, in which the first and second logic voltages are opposite toeach other.

The invention provides an embodiment of an access method for a staticrandom access memory device, wherein the static random access memorydevice comprises a memory cell coupled to a read word line and a writeword line, a read bit line, a write bit line and a complementary writebit line. The method comprises activating the read word line during afirst time interval, applying first and second logic voltagesrepresenting an input signal from a data driver to the write bit lineand the complementary write bit line, respectively, in the first timeinterval, and activating the write word line in the first time intervalto write the first and second logic voltages on the write bit line andthe complementary write bit line to the memory cell.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention can be more fully understood by reading the subsequentdetailed description and examples with references made to theaccompanying drawings, wherein:

FIG. 1 is a schematic diagram of an embodiment of a static random access(SRAM);

FIG. 2 shows another embodiment of the SRAM;

FIG. 3 is a timing chart illustrating a read cycle during an accessmethod for the SRAM;

FIG. 4 is a timing chart illustrating a write cycle during an accessmethod for the SRAM;

FIG. 5 shows another embodiment of the SRAM; and

FIG. 6 shows another embodiment of the SRAM.

DETAILED DESCRIPTION OF THE INVENTION

The following description is of the best-contemplated mode of carryingout the invention. This description is made for the purpose ofillustrating the general principles of the invention and should not betaken in a limiting sense. The scope of the invention is best determinedby reference to the appended claims.

FIG. 1 is a schematic diagram of an embodiment of a static random accessmemory device (SRAM) 100A mainly comprising memory cells BC1 and BC2,write bit lines WBL1, WBL1 , WBL2 and WBL2 , read bit lines RBL1 andRBL2, a read word line RWL, a write word line WWL, and a word linedriving unit 10. The memory cells BC1 and BC2 each can store a logicstate of a bit, in which the memory cell BC1 comprises transistorsM1˜M8, and the memory cell BC2 comprises transistor M9˜M16, and thememory cells BC1 and BC2 are also referred to as an 8T SRAM memory cell.

The transistor M1 comprises a first terminal coupled to the write bitline WBL1, a second terminal coupled to a node N1, and a controlterminal coupled to the write word line WWL. The transistor M2 comprisesa first terminal coupled to a power voltage Vdd, a second terminalcoupled to the node N1, and a control terminal coupled to a node N2. Thetransistor M3 comprises a first terminal coupled to the node N1, asecond terminal coupled to a ground voltage Gnd, and a control terminalcoupled to the node N2. The transistor M4 comprises a first terminalcoupled to the power voltage Vdd, a second terminal coupled to the nodeN2, and a control terminal coupled to the node N1.

The transistor M5 comprises a first terminal coupled to the node N2, asecond terminal coupled to the ground voltage Gnd, and a controlterminal coupled to the node N1. The transistor M6 comprises a firstterminal coupled to the node N2, a second terminal coupled to the writebit line WBL1 , and a control terminal coupled to the write word lineWWL. The transistor M2˜M5 are connected to implement a latch for storinga state of a bit. The transistor M7 comprises a first terminal coupledto the transistor M8, a second terminal coupled to the ground voltageGnd, and a control terminal coupled to the node N2. The transistor M8comprises a first terminal coupled to the read bit line RBL1, a secondterminal coupled to the transistor M7, and a control terminal coupled tothe read word line RWL. For example, the transistors M2 and M3 areimplemented as an inverter and the transistors M4 and M5 are implementedas another inverter and the transistors M2˜M5 can be regarded as alatch.

The transistor M9 comprises a first terminal coupled to the write bitline WBL2, a second terminal coupled to a node N3, and a controlterminal coupled to the write word line WWL. The transistor M10comprises a first terminal coupled to the power voltage Vdd, a secondterminal coupled to the node N3, and a control terminal coupled to anode N4. The transistor M11 comprises a first terminal coupled to thenode N3, a second terminal coupled to the ground voltage Gnd, and acontrol terminal coupled to the node N4. The transistor M12 comprises afirst terminal coupled to the power voltage Vdd, a second terminalcoupled to the node N4, and a control terminal coupled to the node N3.

The transistor M13 comprises a first terminal coupled to the node N4, asecond terminal coupled to the ground voltage Gnd, and a controlterminal coupled to the node N3. The transistor M14 comprises a firstterminal coupled to the node N4, a second terminal coupled to the writebit line WBL2 , and a control terminal coupled to the write word lineWWL. The transistors M10˜M13 are connected to implement a latch forstoring a state of a bit. The transistor M15 comprises a first terminalcoupled to the transistor M16, a second terminal coupled to the groundvoltage Gnd, and a control terminal coupled to the node N4. Thetransistor M16 comprises a first terminal coupled to the read bit lineRBL2, a second terminal coupled to the transistor M15, and a controlterminal coupled to the read word line RWL. For example, the transistorsM10 and M11 are implemented as an inverter and the transistors M12 andM13 are implemented as another inverter and the transistors M10˜M13 areregarded as a latch.

Operations of the memory cell BC1 during a read cycle is discussedhereinafter. First, the read word line RWL is activated, for example ispulled to logic high (i.e., the power voltage Vdd), the read bit lineRBL1 maintains at logic high or is pulled to logic low (i.e., the groundvoltage Gnd) according to the state stored in the latch (i.e.,transistors M2˜M5). For example, the read bit line RBL1 maintains atlogic high when the logic state stored in the memory cell BC1 is logichigh, i.e., the logic voltage level on the node N1 is at logic high andthe logic voltage level on the node N2 is at logic low. On the contrary,the read bit line RBL1 is pulled to logic low when the logic statestored in memory cell BC1 is logic low, i.e., the logic voltage level onthe node N1 is at logic low and the logic voltage level on the node N2is at logic high. Operations of the memory cell BC2 during the readcycle is similar to that of the memory cell BC1, and thus are omittedfor brevity.

Operations of the memory cell BC1 during a write cycle is discussedhereinafter. First, the write bit lines WBL1 and WBL1 are pulled tologic high and logic low, respectively, and then the word line drivingunit 10 activates the write word line WWL (i.e., pulls the write wordline WWL to logic high). Accordingly, the transistors M1 and M6 areturned on, and the node N1 and the node N2 are pulled to logic high andlogic low, respectively, i.e., the latch constructed by transistorsM2˜M5 stores logic “1” of a bit. On the contrary, when the write bitlines WBL1 and WBL1 are pulled to logic low and logic high,respectively, the node N1 and the node N2 are pulled to logic low andlogic high, respectively, i.e., the latch constructed by transistorsM2˜M5 stores logic “0” of a bit.

Because write bit lines WBL2 and WBL2 are pulled to logic high duringactivation of the write word line to write the memory cell BC1, thelogic state stored in the memory cell BC2 may be distorted (i.e.,stability issues, such as static noise margin issues) due tosemiconductor process variation.

FIG. 2 shows another embodiment of an SRAM. As shown, the SRAM 100B issimilar to the SRAM 100A shown in FIG. 1, differing only, in that twomultiplexing units MUA1 and MUA2 are added to prevent stability issuescaused by the write operation. Components and connection of the memorycells BC1 and BC2 are similar to that shown in FIG. 1, and thus areomitted for brevity. The multiplexing unit MUA1 is coupled to bit wordlines WBL1 and WBL1 , read bit line RBL1 and a selection signal SEL1,and the multiplexing unit MUA2 is coupled to bit word lines WBL2 andWBL2 , read bit line RBL2 and a selection signal SEL2.

The multiplexing units MUA1 maintains the write bit lines WBL1 and WBL1at opposite logic voltage levels, and the multiplexing units MUA2maintains the write bit lines WBL2 and WBL2 at opposite logic voltagelevels.

In addition, the multiplexing units MUA1 determines the voltage level atthe bit word lines WBL1 and WBL1 according to the input signal DIN1representing a logic state from a data driver (not shown) or the logicstate stored in the memory cell BC1. Similarly, the multiplexing unitsMUA2 determines the logic voltage level at the bit word lines WBL2 andWBL2 according to the input signal DIN2 representing a logic state fromanother data driver (not shown) or the logic state stored in the memorycell BC1.

FIG. 3 shows a timing chart illustrating a read cycle of the SRAMaccording to the invention. Detailed operations of the SRAM 100B arediscussed hereinafter with reference to FIGS. 2 and 3. During a readcycle RC, the word line driving unit 10 (shown in FIGS. 1 and 2)activates (i.e., pulls high) the read word line RWL, and then the readbit line RBL1 maintains at logic high or is pulled to logic lowaccording to the logic state stored in the memory cell BC1, and the readbit line RBL2 maintains at logic high or is pulled to logic lowaccording to the logic state stored in the memory cell BC2.

For example, after the read word line RWL is activated, the read bitline RBL1 is pulled to logic low and the read bit line RBL2 ismaintained at logic high when the logic state stored in the memory cellBC1 is logic low (i.e., the logic voltage level on the node N1 is atlogic low and the logic voltage level on the node N2 is at logic high)and that stored in the memory cell BC2 is logic high (i.e., the logicvoltage level on the node N3 is at logic high and the logic voltagelevel on the node N4 is at logic low), as shown in FIG. 3.Alternatively, the read bit line RBL1 is maintained at logic high andthe read bit line RBL2 is pulled to logic low when the logic statestored in the memory cell BC1 is logic high (i.e., the logic voltagelevel on the node N1 is at logic high and the logic voltage level on thenode N2 is at logic low) and that stored in the memory cell BC2 is logiclow (i.e., the logic voltage level on the node N3 is at logic low andthe logic voltage level on the node N4 is at logic high). Further, thelogic states stored in the memory cell BC1 and BC2 are both logic highif the read bit lines RBL1 and RBL2 are both maintained at logic high.Moreover, if the read bit lines RBL1 and RBL2 are both pulled to logiclow, the logic states stored in the memory cell BC1 and BC2 are bothlogic low.

FIG. 4 shows a timing chart illustrating a write cycle of the SRAMaccording to the invention. Detailed operations of the SRAM 100B arediscussed hereinafter with reference to FIGS. 2 and 4. During a writecycle WC for writing the memory cell BC2, as the selection signals SEL1and SEL2 are deactivated and activated, respectively, the multiplexingunit MUA1 determines logic voltage levels at the write bit lines WBL1and WBL1 according to the logic state stored in the memory cell BC1 andthe multiplexing unit MUA2 determines logic voltage levels at the writebit lines WBL2 and WBL2 according to the input signal DIN2.

Specifically, the word line driving unit 10 first activates the readword line RWL during the write cycle WC, the read bit line RBL1 ispulled to logic low when the logic state stored in the memory cell BC1is logic low, and the multiplexing unit MUA1 accordingly pulls the writebit lines WBL1 and WBL1 to logic low and logic high, respectively. Onthe contrary, the multiplexing unit MUA1 pulls the write bit lines WBL1and WBL1 to logic high and logic low, respectively, if the logic statestored in the memory cell BC1 is logic high. Namely, the write bit linesWBL1 would be pulled to logic low when the logic state stored in thememory cell BC1 is logic low, and the write bit lines WBL1 would bepulled to logic high when the logic state stored in the memory cell BC1is logic high.

At the same time, the multiplexing unit MUA2 pulls the write bit linesWBL2 and WBL2 to logic low and logic high, respectively, when the inputsignal DIN2 represents a low logic state, because the selection signalSEL2 is activated. On the contrary, the multiplexing unit MUA2 pulls thewrite bit lines WBL2 and WBL2 to logic high and logic low, respectivelyif the input signal DIN2 represents a high logic state.

Next, the word line driving unit 10 activates the write word line WWL,the transistors M9 and M14 are turned on, and the nodes N3 and N4 arepulled to logic high and logic low, respectively, i.e., the input signalDIN2 representing a high logic state is written to the memory cell BC2.When the write word line is activated, the transistors M1 and M6 arealso turned on, the logic state stored in the memory cell BC1 ismaintained at logic low because the write bit lines WBL1 and WBL1 arelogic low and logic high, respectively and the nodes N1 and N2 are logiclow and logic high, respectively.

Afterwards, the word line driving unit 10 deactivates (i.e., pulls low)the write word line WWL and the read word line in sequence, and then theselection signal SEL2 is deactivated. Before ending a write cycle WC,the read bit lines RBL1 and RBL2 are both pulled to logic high, andthus, the multiplexing unit MUA1 pulls the write bit lines WBL1 and WBL1to logic high and logic low, respectively and the multiplexing unit MUA2pulls the write bit lines WBL2 and WBL2 to logic high and logic low,respectively.

Similarly, during a write cycle for writing the memory cell BC1, as theselection signals SEL1 and SEL2 are activated and deactivated,respectively, the multiplexing unit MUA1 determines voltage levels atthe write bit lines WBL1 and WBL1 according to the input signal DIN1 andthe multiplexing unit MUA2 determines voltage levels at the write bitlines WBL2 and WBL2 according to the logic state stored in the memorycell BC2. The detailed operations of the SRAM 100B during the writecycle for writing the memory cell BC1 are similar to that for writingthe BC2 described above and thus are omitted for brevity.

Namely, when the memory cell BC1/BC2 is selected to be written by aninput signal representing a logic state from a data driver, the logicstate stored in the unselected memory cell BC2/BC1 is maintained by thecorresponding multiplexing units. Hence, the logic state stored in theunselected memory cell BC2/BC1 is not distorted due to semiconductorprocess variation. It should be noted that, because the multiplexingunits MUA1 and MUA2 can prevent from distortion caused by semiconductorprocess variation during write cycle, the word line driving unit 10activates the write word line WWL by applying the power voltage Vdd or avoltage which is higher than the power voltage Vdd to the write wordline WWL thereby enhancing write speed and success rate in write.

FIG. 5 shows another embodiment of an SRAM. As shown, the SRAM 100C issimilar to the SRAM 100B shown in FIG. 2, differing only, in that themultiplexing unit MUB1 is implemented by an inverter INV1 and twotri-state buffers TRB1 and TRB2, and the multiplexing unit MUB2 isimplemented by an inverter INV4 and two tri-state buffers TRB3 and TRB4.

The inverter INV1 comprises an input terminal coupled to the write bitline WBL1 and an output terminal coupled to the write bit line WBL1. Thetri-state buffer TRB1 comprises an input terminal coupled to the readbit line RBL1, an output terminal coupled to the write bit line WBL1 anda control terminal coupled to the selection signal SEL1. The tri-statebuffer TRB2 comprises an input terminal coupled to the read bit lineDIN1, an output terminal coupled to the write bit line WBL1 and acontrol terminal coupled to the selection signal SEL1. The inverter INV4comprises an input terminal coupled to the write bit line WBL2 and anoutput terminal coupled to the write bit line WBL2. The tri-state bufferTRB3 comprises an input terminal coupled to the read bit line RBL2, anoutput terminal coupled to the write bit line WBL2 and a controlterminal coupled to the selection signal SEL2. The tri-state buffer TRB4comprises an input terminal coupled to the read bit line DIN2, an outputterminal coupled to the write bit line WBL2 and a control terminalcoupled to the selection signal SEL2.

Detailed operations of the SRAM 100C during a read cycle are similar tothat of the SRAM 100B shown in FIG. 2 and thus are omitted for brevity.Detailed operations of the SRAM 100C during a write cycle are discussedhereinafter with reference to FIGS. 4 and 5.

First, the word line driving unit 10 (shown in FIGS. 1 and 2) activatesthe read word line RWL during the write cycle WC, and the tri-statebuffer TRB2 in the multiplexing unit MUB1 is disabled when the selectionsignal SEL1 is deactivated, such that the voltage levels on the writebit lines WBL1 and WBL1 are determined according to the state stored inthe memory cell BC1. For example, the read bit line RBL1 is pulled tologic low when the logic state stored in the memory cell BC1 is logiclow. Consequently, the tri-state buffer TRB1 pulls the write bit lineWBL1 to logic high and the inverter INV1 pulls the write bit lines WBL1to logic low. On the contrary, the tri-state buffer TRB1 pulls the writebit line WBL1 to logic low and the inverter INV1 pulls the write bitlines WBL1 to logic high if the logic state stored in the memory cellBC1 is logic high.

At the same time, the tri-state buffer TRB3 in the multiplexing unitMUB2 is disabled because the selection signal SEL2 is activated, suchthat the voltage levels of the write bit lines WBL2 and WBL2 aredetermined according to the input signal DIN2. For example, thetri-state buffer TRB4 pulls the write bit lines WBL2 to logic high andthe inverter INV4 pulls the write bit line WBL2 to logic low when theinput signal DIN2 is logic low. On the contrary, the tri-state bufferTRB4 pulls the write bit line WBL2 to logic low and the inverter INV4pulls the write bit line WBL2 to logic high if the input signal DIN2 islogic high.

Next, the word line driving unit 10 activates the write word line WWL,the transistors M9 and M14 in memory cell BC2 are turned on, and thenodes N3 and N4 are pulled to logic high and logic low, respectively,i.e., the input signal DIN2 is written to the memory cell BC2. At thesame time, the transistors M1 and M6 in memory cell BC1 are also turnedon when write word line WWL is activated. Because the write bit linesWBL1 and WBL1 are logic low and logic high, respectively and the nodesN1 and N2 are logic low and logic high, respectively, the logic statestored in the memory cell BC1 is maintained at logic low.

Afterwards, the word line driving unit 10 deactivates the write wordline WWL and the read word line RWL in sequence, and then the selectionsignal SEL2 is deactivated and the read bit lines RBL1 and RBL2 are bothpulled to logic high. At this time, because the selection signals SEL1and SEL2 are both deactivated and the read bit lines RBL1 and RBL2 areboth pulled to logic high, the tri-state buffers TRB1 and TRB3 pull thewrite bit lines WBL1 and WBL2 to logic low, and the inverters INV1 andINV4 pull the write bit lines WBL1 and WBL2 to logic high. The detailedoperations of the SRAM 100C during the write cycle when the selectionsignals SEL1 and SEL2 are activated and deactivated, respectively, aresimilar to that described above and thus are omitted for brevity.

Hence, when the memory cell BC1/BC2 is selected to be written by theinput signal, the state stored in the unselected memory cell BC2/BC1 ismaintained by the corresponding multiplexing units. Thus, the statestored in the unselected memory cell BC2/BC1 is not distorted due tosemiconductor process variation.

FIG. 6 shows another embodiment of an SRAM. As shown, the SRAM 100D issimilar to the SRAM 100A shown in FIG. 1, differing only, in that themultiplexing unit MUC1 is implemented by transistors M17˜M26 and themultiplexing unit MUC2 is implemented by transistors M27˜M36. Thetransistors M17 and M18 form an inverter, and the transistors M19˜M26form two tri-state buffers. The transistor M17 comprises a firstterminal coupled to the power voltage Vdd, a second terminal coupled tothe write bit line WBL1, and the control terminal coupled to the writebit line WBL1 , and the transistor M18 comprises a first terminalcoupled to the write bit line WBL1, a second terminal coupled to theground voltage Gnd, and the control terminal coupled to the write bitline WBL1 .

The transistor M19 comprises a first terminal coupled to a power voltageVdd, a control terminal coupled to a selection signal YSEL1 and a secondterminal coupled to the transistor M20. The transistor M20 comprises afirst terminal coupled to the second terminal of the transistor M19, asecond terminal coupled to the write bit line WBL1 and a controlterminal coupled to the read bit line RBL1. The transistor M21 comprisesa first terminal coupled to the write bit line WBL1 , a second terminalcoupled to the transistor M22 and a control terminal coupled to the readbit line RBL1. The transistor M22 comprises a first terminal coupled toa second terminal of the transistor M21, a control terminal coupled to acomplementary signal YSEL1 of the selection signal YSEL1 and a secondterminal coupled to the ground voltage Gnd.

The transistor M23 comprises a first terminal coupled to a power voltageVdd, a control terminal coupled to the complementary signal YSEL1 and asecond terminal coupled to the transistor M24. The transistor M24comprises a first terminal coupled to the second terminal of thetransistor M23, a second terminal coupled to the write bit line WBL1 anda control terminal coupled to the input signal DIN1. The transistor M25comprises a first terminal coupled to the write bit line WBL1 , a secondterminal coupled to the transistor M26 and a control terminal coupled tothe input signal DIN1. The transistor M26 comprises a first terminalcoupled to a second terminal of the transistor M25, a control terminalcoupled to the selection signal YSEL1 and a second terminal coupled tothe ground voltage Gnd.

The transistor M27 comprises a first terminal coupled to the powervoltage Vdd, a second terminal coupled to the write bit line WBL2, andthe control terminal coupled to the write bit line WBL2 , and thetransistor M28 comprises a first terminal coupled to the write bit lineWBL2, a second terminal coupled to the ground voltage Gnd, and thecontrol terminal coupled to the write bit line WBL2 .

The transistor M29 comprises a first terminal coupled to a power voltageVdd, a control terminal coupled to a selection signal YSEL2 and a secondterminal coupled to the transistor M30. The transistor M30 comprises afirst terminal coupled to the second terminal of the transistor M29, asecond terminal coupled to the write bit line WBL2 and a controlterminal coupled to the read bit line RBL2. The transistor M31 comprisesa first terminal coupled to the write bit line WBL2 , a second terminalcoupled to the transistor M32 and a control terminal coupled to the readbit line RBL2. The transistor M32 comprises a first terminal coupled toa second terminal of the transistor M31, a control terminal coupled to acomplementary signal YSEL2 of the selection signal YSEL2 and a secondterminal coupled to the ground voltage Gnd.

The transistor M33 comprises a first terminal coupled to a power voltageVdd, a control terminal coupled to the complementary signal YSEL2 and asecond terminal coupled to the transistor M34. The transistor M34comprises a first terminal coupled to the second terminal of thetransistor M33, a second terminal coupled to the write bit line WBL2 anda control terminal coupled to the input signal DIN2. The transistor M35comprises a first terminal coupled to the write bit line WBL2 , a secondterminal coupled to the transistor M36 and a control terminal coupled tothe input signal DIN2. The transistor M36 comprises a first terminalcoupled to a second terminal of the transistor M35, a control terminalcoupled to the selection signal YSEL2 and a second terminal coupled tothe ground voltage Gnd.

Detailed operations of the SRAM 100D during a read cycle are similar tothat of the SRAM 100B shown in FIG. 2 and thus are omitted for brevity.Detailed operations of the SRAM 100D during a write cycle for writingmemory cell BC2 are discussed hereinafter.

At first, the word line driving unit 10 (shown in FIGS. 1 and 2)activates the read word line RWL during the write cycle WC, and thetri-state buffer formed by transistors M23˜M26 is disabled because theselection signal YSEL1 and its complementary signal YSEL1 are logic lowand logic high (i.e., the selection signal YSEL1 is deactivated), suchthat the voltage levels of the write bit lines WBL1 and WBL1 aredetermined according to the logic state stored in the memory cell BC1.

For example, the read bit line RBL1 is pulled to logic low when thelogic state stored in the memory cell BC1 (i.e., the voltage level atthe node N1) is logic low, and thus, the transistors M19 and M20 pullthe write bit lines WBL1 to logic high and the transistor M18 pulls thewrite bit line WBL1 to logic low. Namely, the multiplexing unit MUC1feedbacks the logic state on the read bit line RBL1 to the write bitline WBL1. On the contrary, if the logic state stored in the memory cellBC1 is high, the transistors M21 and M22 pull the write bit line WBL1 tologic low and the transistor M17 pulls the write bit line WBL1 to logichigh. Namely, the multiplexing unit MUC1 feedbacks the logic state onthe read bit line RBL1 to the write bit line WBL1.

At the same time, the tri-state buffer formed by the transistors M29˜M32is disabled because the selection signal YSEL2 and its complementarysignal YSEL2 are logic high and logic low (i.e., the selection signalYSEL2 is activated), such that the voltage levels of the write bit linesWBL2 and WBL2 are determined according to the input signal DIN2. Forexample, the transistors M33 and M34 pull the write bit line WBL2 tologic high and the transistor M28 pulls the write bit line WBL2 to logiclow when the input signal DIN2 represents a logic low state. On thecontrary, if the input signal DIN2 represents a logic high state, thetransistors M35 and M36 pull the write bit line WBL2 to logic low andthe transistor M27 pulls the write bit line WBL2 to logic high.

Next, the word line driving unit 10 activates the write word line WWL,the transistors M9 and M14 in memory cell BC2 are turned on, and thenodes N3 and N4 are pulled to logic high and logic low, respectively,i.e., the input signal DIN2 representing a logic low state is written tothe memory cell BC2. At the same time, the transistors M1 and M6 inmemory cell BC1 are turned on when the write word line WWL is activated,the logic state stored in the memory cell BC1 is maintained at logiclow, because the nodes N1 and N2 are logic low and logic highrespectively, and the write bit lines WBL1 and WBL1 are logic low andlogic high respectively.

Afterwards, the word line driving unit 10 activates the write word lineWWL and the read word line RWL in sequence, and then the selectionsignal YSEL2 is pulled to logic low and the complementary signal YSEL2is pulled to logic high (i.e., the selection signal SEL2 isdeactivated), and the read bit lines RBL1 and RBL2 are both pulled tologic high. At this time, because the selection signals SEL1 and SEL2are both deactivated and the read bit lines RBL1 and RBL2 are bothpulled to logic high, the transistors M21 and M22 pull the write bitline WBL1 to logic low, the transistors M31 and M32 pull the write bitline WBL2 to logic low, the transistor M17 pulls the write bit line WBL1to logic high, and the transistor M27 pulls the write bit line WBL2 tologic high. The detailed operations of the SRAM 100C during the writecycle when the selection signals SEL1 and SEL2 are activated anddeactivated, respectively, are similar to that described above and thusare omitted for brevity.

Thus, when the memory cell BC1/BC2 is selected to be written by a logicstate from a data driver, the state stored in the unselected memory cellBC2/BC1 is maintained by the corresponding multiplexing units, such thatthe state stored in the unselected memory cell BC2/BC1 is not distorteddue to semiconductor process variation during a writing cycle.

The invention also discloses an access method for an SRAM. Readoperations of the access method are discussed hereinafter with referenceto FIGS. 2 and 3. During a read cycle RC, the word line driving unit 10activates the read word line RWL (i.e., pulls the read word line RWL tologic high), and then the read bit lines RBL1 maintains logic high or ispulled to logic low according to the logic states stored in the memorycells BC1 and BC2.

For example, after the read word line RWL is activated, if the read bitlines RBL1 and RBL2 are pulled to logic low and maintained at logic highrespectively, the logic state stored in the memory cell BC1 is logic lowand the logic state stored in the memory cell BC2 is logic high.Alternatively, the logic state stored in the memory cell BC1 is logichigh and the logic state stored in the memory cell BC2 is logic low, ifthe read bit lines RBL1 and RBL2 are maintained at logic high and pulledto logic low respectively. Further, the logic states stored in thememory cell BC1 and BC2 are both logic high if the read bit lines RBL1and RBL2 are both maintained at logic high. Moreover, if the read bitlines RBL1 and RBL2 are both pulled to low, the logic states stored inthe memory cell BC1 and BC2 are both logic low.

Write operations of the access method are discussed hereinafter withFIGS. 2 and 4. During a write cycle WC, as the selection signal SEL2corresponding to the memory cell BC2 is deactivated and the otherselection signal, such as SEL1, is deactivated, the memory cell isselected to be written by an input signal DIN2, i.e., a logic state froma data driver. Then, the word line driving unit 10 activates the readword line RWL, such that the multiplexing unit MUA1 corresponding to theunselected memory cell, such as BC1, determines voltage levels at thewrite bit lines WBL1 and WBL1 according to the logic state stored in thememory cell BC1 and the multiplexing unit MUA2 corresponding to theselected memory cell BC2 determines voltage levels on the write bitlines WBL2 and WBL2 according to the input signal DIN2.

For example, the read bit line RBL1 is pulled to logic low when thelogic state stored in the memory cell BC1 is logic low, and themultiplexing unit MUA1 pulls the write bit lines WBL1 and WBL1 to logiclow and logic high, respectively. On the contrary, the multiplexing unitMUA1 pulls the write bit lines WBL1 and WBL1 to logic high and logiclow, respectively if the logic state stored in the memory cell BC1 islogic high. At the same time, the multiplexing unit MUA2 pulls the writebit lines WBL2 and WBL2 to logic low and logic high, respectively, whenthe input signal DIN2 is logic low. On the contrary, the multiplexingunit MUA2 pulls the write bit lines WBL2 and WBL2 to logic high andlogic low, respectively if the input signal DIN2 is logic high.

Next, the word line driving unit 10 activates the write word line WWL,the transistors M9 and M14 are turned on, and the nodes N3 and N4 arepulled to logic high and logic low, respectively, i.e., the input signalDIN2 representing a logic low state is written to the selected memorycell BC2. At the same time, the transistors M1 and M6 are turned on whenthe write word line WWL is activate, the logic state stored in theunselected memory cell BC1 is maintained at logic low because the nodesN1 and N2 are logic low and logic high, respectively and the write bitlines WBL1 and WBL1 are logic low and logic high, respectively.

Afterwards, the word line driving unit 10 activates the write word lineWWL and the read word line RWL in sequence, and then the selectionsignal SEL2 is deactivated and the read bit lines RBL1 and RBL2 arepulled to logic high. Finally, the multiplexing units MUA1 and MUA2 pullthe write bit lines WBL1 and WBL2 to logic high and pull the write bitlines WBL1 and WBL2 to logic low because the read bit lines RBL1 andRBL2 are pulled to logic high.

It should be noted that the read word line RWL is activated before thewrite word line WWL and is deactivated after the write word line WWL.Namely, the read word line RWL is activated during a first period andthe write word line WWL is activated during a second period within thefirst period. In some embodiments, the read word line RWL and the writeword line WWL can also be deactivated at same time.

Because the multiplexing unit MUA1 feedbacks the logic voltagesrepresenting the logic state stored in the memory cells BC1 to the writebit lines WBL1 and WBL1 before activating the write word line WWL, thelogic state stored in the memory cell BC11 is maintained during a memorycell BC2 writing cycle. Namely, when a particular memory cell isselected to be written by an input signal from a data driver, the logicstate(s) stored in the unselected memory cell(s) which is/are connectedto the same write word line as the selected memory cell is maintained bythe corresponding multiplexing unit(s). Thus, the logic states stored inthe unselected memory cells is not distorted due to semiconductorprocess variation. Although operations of the access method areillustrated with reference to single port SRAM shown in FIGS. 4, 5 and6, the access method can also be applied on two port SRAM.

Certain terms are used throughout the description and claims to refer toparticular system components. As one skilled in the art will appreciate,consumer electronic equipment manufacturers may refer to a component bydifferent names. This document does not intend to distinguish betweencomponents that differ in name but not function.

Although the invention has been described in terms of preferredembodiment, it is not limited thereto. Those skilled in the art can makevarious alterations and modifications without departing from the scopeand spirit of the invention. Therefore, the scope of the invention shallbe defined and protected by the following claims and their equivalents.

1. A static random access memory device, comprising: a first memory cellcoupled to a read word line, a write word line, a first read bit line, afirst write bit line and a first complementary write bit line; a secondmemory cell coupled to the read word line, the write word line, a secondread bit line, a second write bit line and a second complementary writebit line; a word line driving unit coupled to the read word line and thewrite word line; and first and second multiplexing units coupled to thefirst and second the memory cells, wherein the word line driving unitactivates the read word line for a first time interval when the secondmemory cell is selected to be written by an input signal from a datadriver and the word line driving unit then activates the write word linein the first time interval to write the input signal from the datadriver to the second memory cell.
 2. The static random access memorydevice as claimed in claim 1, wherein the first multiplexing unitmaintains a logic state stored in the first memory cell according to avoltage level on the first read bit line, when the word line drivingunit activates the read word line.
 3. The static random access memorydevice as claimed in claim 1, wherein, when the word line driving unitactivates the read word line, a first logic voltage representing a logicstate stored in the first memory cell is output to the first read bitline and the first multiplexing unit feedbacks the first logic voltageon the first read bit line to the first write bit line.
 4. The staticrandom access memory device as claimed in claim 3, wherein the firstmultiplexing unit further applies a second logic voltage to the firstcomplementary write bit line when the read word line is activated,wherein the second logic voltage is opposite to the first logic voltage.5. The static random access memory device as claimed in claim 4, whereinthe second multiplexing unit applies third and fourth logic voltagesrepresenting the input signal from the data driver to the second writebit line and the complementary write bit line respectively before theword line driving unit activates the write word line.
 6. The staticrandom access memory device as claimed in claim 5, wherein the third andfourth logic voltages on the second write bit line and the secondcomplementary write bit line are written to the second memory cell, andthe logic state stored in the first memory cell is maintained by thefirst and second logic voltages on the first write bit line and thefirst complementary write bit line, when the word line driving unitactivates the write word line.
 7. The static random access memory deviceas claimed in claim 1, wherein the word line driving unit deactivatesthe write word line and the read word line in sequence.
 8. The staticrandom access memory device as claimed in claim 1, wherein, the wordline driving unit activates the write word line by applying a powervoltage of the static random access memory or a voltage which is higherthan the power voltage, thereby enhancing write speed and success ratein write.
 9. An access method for a static random access memory device,wherein the static random access memory device comprises first andsecond memory cells driven by a read word line and a write word line,and the first memory cell is further coupled to a first read bit line, afirst write bit line and a first complementary write bit line and thesecond memory cell is further coupled to a second read bit line, asecond write bit line and a second complementary write bit line, themethod comprising: activating the read word line for a first timeinterval when the second memory cell is selected to be written by aninput signal from a data driver; and activating the write word line towrite the input signal from the data driver to the second memory cellduring the first time interval.
 10. The access method as claimed inclaim 9, further comprising maintaining a logic state stored in thefirst memory according to a voltage level on the read bit line when thewrite word line is activated.
 11. The access method as claimed in claim9, further comprising outputting a first logic voltage representing alogic state stored in the first memory cell to the first read bit linewhen the read word line is activated.
 12. The access method as claimedin claim 11, further comprising outputting a second logic voltagerepresenting the logic state stored in the first memory cell to thefirst complementary write bit line when the read word line is activated,wherein the second logic voltage is opposite to the first logic voltage.13. The access method as claimed in claim 11, further comprisingdetermining voltage levels on the second write bit line and the secondcomplementary write bit line according to the input signal before thewrite word line is activated.
 14. The access method as claimed in claim9, wherein the write word line is activated after the read word line inthe first time interval.
 15. The access method as claimed in claim 9,further comprising deactivating the write word line and the read wordline in sequence.
 16. The access method as claimed in claim 9, whereinthe write word line is activated by a power voltage of the static randomaccess memory or a voltage which is higher than the power voltage,thereby enhancing write speed and success rate in write.